Index: webform.module
===================================================================
RCS file: /cvs/drupal/contributions/modules/webform/webform.module,v
retrieving revision 1.196.2.52
diff -u -r1.196.2.52 webform.module
--- webform.module	30 Aug 2010 20:22:15 -0000	1.196.2.52
+++ webform.module	1 Sep 2010 08:55:14 -0000
@@ -1724,7 +1724,7 @@
             }
           }
         }
-        elseif (!isset($options[$elements['#value']])) {
+        elseif ($elements['#value'] != '' && !isset($options[$elements['#value']])) {
           form_error($elements, t('An illegal choice has been detected. Please contact the site administrator.'));
           watchdog('form', 'Illegal choice %choice in %name element.', array('%choice' => $elements['#value'], '%name' => empty($elements['#title']) ? $elements['#parents'][0] : $elements['#title']), WATCHDOG_ERROR);
         }